The location of the factory installed Lowrance transom transducer is the best place if you want on-plane depth readings with the SI transducer.
Tracker finally installed the transducer in the proper place for that stepped transom style for the 2012 year model after many years of doing it wrong.
It will be subject to possible impact damage though.

If I was you, I would keep the factory unit for high speed depth and install the SI transducer above the hull bottom just under the right side of the outboard motor mount bolt with the bottom of the transducer mount even with the bottom of the transom. That will have the transducer itself hanging below the step so you can scan at up to about 4-5 mph and be somewhat protected from impact damage.

I don't have a picture of the transom right side of the 2012 year model (I do have a left side showing the factory transducer) but this is one I did for an earlier model. The black box is the mount and the yellow line is the transducer.

I have a 2003 tracker pt185. I mounted the transducer in the spot you have marked with the yellow line. I could not get a reading when going faster than 10 mph. I moved it directly below that spot under transom step down. it works perfect at any speed. I thought it might pick up some prop wash or interference but nothing at all affects it. Hope this helps.

I have a 2003 tracker pt185. I mounted the transducer in the spot you have marked with the yellow line. I could not get a reading when going faster than 10 mph. I moved it directly below that spot under transom step down. it works perfect at any speed. I thought it might pick up some prop wash or interference but nothing at all affects it. Hope this helps.

You cannot get "prop wash" with an outboard and transom mounted transducer unless the motor is in reverse.